I've used a Speedy-Stitcher for years. Great on the heavy and straightforward jobs. I used it to restitch the front section (over the lens) of a Leica case. Best where you are working straight through a relatively flat workpiece.
To repair (replace) the stitching on my Rolleiflex case and the case for my grandfather's Zeiss binoculars, I had to work on the corner stitching where the panels join at 90 degrees. For that work I found it easier to use two plain (straight or curved) needles each with it's own thread (or working both ends of a single length of thread). Rather than trying for a lock-stitch (as with the Speedy-Stitcher) use a simple stitch, doubled, by working through each of the original holes from both sides. If you work carefully you won't damage the leather or tear the original holes. Gently tension each stitch from both sides as you go. A pair of long-nose pliers is essential for holding the needles.